Acid-Base Balance and Blood Gases PDF

Acid-Base Balance and Blood Gases Learning objectives Review of pH balance Classify acid base disorders Describe pH imbalance in acid base disorders Describe the role of blood gases in acid-base balance  pH Review Acids Ionic compounds (positive or a ne...

Acid-Base Balance and Blood Gases Learning objectives Review of pH balance Classify acid base disorders Describe pH imbalance in acid base disorders Describe the role of blood gases in acid-base balance  pH Review Acids Ionic compounds (positive or a negative charge) that dissociates in water to form a hydrogen ion (H+). Acids are H+ donors. The strength of an acid is based on the concentration of H+ ions in solution. The higher the [H+] the stronger the acid; e.g HCl in water Bases Bases are ionic compounds that dissociate to form negatively charged hyroxide ion (OH-). Bases are H+ acceptors, or give up OH- in solution The greater the concentration of OH- ions the stronger the base e.g NaOH in water or low [H+]  pH Review continued…… Neutralisation reaction When acids and bases are added to each other they react to neutralize each other if an equal number of hydrogen and hydroxide ions are present. When this reaction occurs -salt and water are formed. HCl + NaOH NaCl + H2O (Acid) (Base)---(Salt) (Water) Acid-Base Balance  The Body and pH Variable Primary disorder Normal Range, Primary disorder Arterial Gas pH Acidaemia 7.35-7.45 Alkalaemia PCO2, mm of Respiratory Respiratory mercury alkalosis 35-45 acidosis Bicarbonate, Metabolic Metabolic mmol/litre acidosis 22-26 alkalosis  Diagnosis using ABGs Acidemia = a decrease in blood pH caused by acidosis Alkalemia = a increase in blood pH caused by alkalosis Normal pH range is 7.35 – 7.45  Diagnosis using ABGs Look at pCO2 Acidaemia + raised pCO2 = Respiratory Acidosis Alkalaemia + low pCO2 = Respiratory Alkalosis  Diagnosis using ABGs Look at HCO3- Alkalaemia + raised HCO3- = Metabolic Alkalosis Acidaemia + low HCO3- = Metabolic Acidosis Chemical buffers e.g bicarbonate, phosphate protein and haemoglobin are the first line of defense against pH shift (instantaneous). Second line of defense against pH shift include physiological buffers e.g Respiratory (CO2 excretion, several minutes to hours) and Renal mechanism (H+ excretion, several hours to days).  H+ gain and loss H+ Gain CO2 in blood (combine with H2O via carbonic anhydrase) Nonvolatile acids from metabolism (e.g. lactic) Loss of HCO - in diarrhoea or non-gastric GI fluids 3 Loss of HCO - in urine 3 H+ Loss Use of H+ in metabolism of organic anions Loss of H+ in vomit Loss of H+ in urine Hyperventilation (blow off CO2)  Acid-Base Imbalances The body response to acid-base imbalance is called compensation May be complete if brought back within normal limits Partial compensation if range is still outside norms. Compensation If underlying problem is metabolic, hyperventilation or hypoventilation can help : respiratory compensation. If problem is respiratory, renal mechanisms can bring about metabolic compensation.  Compensation CO2 retention causing acidaemia Secretion of H+ ions Retention of HCO3 Acid-Base Imbalances  Respiratory Acidosis Carbonic acid excess caused by blood levels of CO2 above 45 mm Hg. Hypercapnia (high levels of CO2 in blood) Chronic conditions: – Depression of respiratory center in brain that controls breathing rate – drugs or head trauma – Paralysis of respiratory or chest muscles – Emphysema Compensation for Respiratory Acidosis Kidneys eliminate hydrogen ion and retain bicarbonate ion Signs and Symptoms of Respiratory Acidosis Breathlessness Restlessness Lethargy and disorientation Tremors, convulsions, coma Respiratory rate rapid, then gradually depressed Skin warm and flushed due to vasodilation caused by excess CO2  Respiratory Acidosis continued…. Treatment of Respiratory Acidosis Restore ventilation IV lactate solution Treat underlying dysfunction or disease  Respiratory Alkalosis Excessive loss of CO2 (hyperventilation) Most common acid-base imbalance Primary cause is hyperventilation Hypocapnia (low levels of CO2 in blood) Respiratory Alkalosis Conditions that stimulate respiratory center: – Oxygen deficiency at high altitudes – Pulmonary disease and Congestive heart failure – caused by hypoxia – Acute anxiety – Early salicylate intoxication – Fever, anemia – Cirrhosis – Gram-negative sepsis Compensation of Respiratory Alkalosis Kidneys conserve hydrogen ion Excrete bicarbonate ion  Respiratory Alkalosis continued… Treatment of Respiratory Alkalosis Treat underlying cause Breathe into a paper bag IV Chloride containing solution – Cl- ions replace lost bicarbonate ions  Metabolic Acidosis Bicarbonate deficit - blood concentrations of bicarb drop below 22mEq/L Causes: – Loss of bicarbonate through diarrhea or renal dysfunction – Accumulation of acids (lactic acid or ketones) – Failure of kidneys to excrete H+ Symptoms of Metabolic Acidosis Headache, lethargy Nausea, vomiting, diarrhea Coma Death Compensation for Metabolic Acidosis Increased ventilation Renal excretion of hydrogen ions if possible K+ exchanges with excess H+ in ECF ( H+ into cells, K+ out of cells) Treatment of Metabolic Acidosis IV lactate solution  Metabolic Alkalosis Bicarbonate excess - concentration in blood is greater than 26 mEq/L Causes: – Excess vomiting = loss of stomach acid – Excessive use of alkaline drugs – Certain diuretics – Endocrine disorders – Heavy ingestion of antacids – Severe dehydration Compensation for Metabolic Alkalosis Alkalosis most commonly occurs with renal dysfunction, so can’t count on kidneys Respiratory compensation difficult – hypoventilation limited by hypoxia Symptoms of Metabolic Alkalosis Respiration slow and shallow Hyperactive reflexes ; tetany Often related to depletion of electrolytes Atrial tachycardia Dysrhythmias Metabolic Alkalosis continued…. Treatment of Metabolic Alkalosis Electrolytes to replace those lost IV chloride containing solution Treat underlying disorder